Question: In chemistry class, the girl to boy ratio is $9$ to $3$. If there are a total of $60$ students, how many girls are there?
A ratio of $9$ girls to $3$ boys means that a set of $12$ students will have $9$ girls and $3$ boys. A class of $60$ students has $5$ sets of $12$ students. Because we know that there are $9$ girls in each set of $12$ students, the class must have $5$ groups of $9$ girls each. There is a total of $45$ girls in chemistry class.